A popular antenna element for ultra-wideband antenna arrays is the Vivaldi aerial (antenna), also commonly known as the tapered-slot or flared-notch antenna. As stand-alone antennas, Vivaldi aerials can exhibit ultra-wide bandwidths of operation, where the high frequency range is typically limited by the slot-width near the feed, and low frequency is typically dictated by the width of the overall taper (must be approximately at least half a wavelength), with the overall length of the taper in the direction of end-fire radiation typically several wavelengths long. They function in a slightly different manner when condensed into an array configuration, where the mutual coupling between adjacent elements creates constructive interference to generate radiation. In an array, the element spacing is roughly half a wavelength at the highest frequency, and the low-frequency limit is set by the overall footprint of the array (comparable again to roughly half a wavelength at the low frequency). This assumes an ideal feed.
When condensed into an array configuration, another low-frequency limitation is introduced by the limited space for feed circuits. It is difficult to design ultrawideband circuits that can fit in the constricted array element cell. For this reason, it is common to find Vivaldi arrays that can achieve operational bandwidths of 3:1, 4:1, and occasionally 5:1, but seldom greater than that. Ferrite-tuned feed circuits can potentially achieve high bandwidths, but cannot be used at microwave frequencies and above (GHz range).
In its most usual form as shown in FIG. 1, a Vivaldi antenna element 10 consists of a body 12 with a tapered slot 14 along the centerline with a flared opening at one end and at the other end terminates in a slot-line open cavity 16. A strip-line or microstrip feed 18 (shown in dotted lines) terminates in a quarter-wave stub 20 necessary for enabling the strip-to-slot feed transition. The antenna 10 guides a wave from transition line impedances (˜50 Ohms) to free space impedances (˜377 Ohms) by gradually changing the taper of the slot 14 until the energy can simply launch from the guided slot into free space. This remarkably simple antenna concept can be used to radiate energy over nearly any range of frequencies that can be guided along the slot-line. Typically, Vivaldi elements are printed on circuit board and fed from micro-strip or strip-line feeds. This is done because it is cheap to manufacture. However, it also creates a number of limitations including stored (reactive) energy and also power-handling problems. Most antennas are fed via a coaxial cable 22. In these embodiments, the coax must transition to micro-strip or strip-line and then to slot-line, hence requiring multiple transitions/transformers that are typically band-limited in design, and therefore limit performance.